1. Field of the Invention
The present invention relates to an electromagnetic device for the measurement of tubular goods wall thickness which includes additional coil sensing structure for defining the complete magnetic field distribution as between the main sensing coil and the tubular goods so that a direct indication of wall thickness can be obtained with the field strength measurements.
2. Description of the Prior Art
Various forms of electromagnetic casing inspection tool have been utilized for a long number of years. There are various types of pad-type electromagnetic tool which utilize a main sensing coil aligned generally axially through the casing in combination with a plurality of pad-type sensing coils maintained in contact with the inner casing wall to sense flux leakage variations thereby to derive a relative indication of any discontinuities or thinning walls in the casing. Examples of such prior casing testing tools are the VERTILOG.TM. system of Dresser Atlas Corporation, and the PIPE ANALYSIS LOG which is a system designed and operated by Schlumberger Corp. In these older types of tool, it was required to have a joint of good casing in order to establish a calibration reference from which subsequent findings of wall thickness might be compared. The drawback in this situation comes from the fact that such casing section cannot usually be found in an old installation having previous corrosion.
The U.S. Pat. No. 2,116,119 in the name of Loewenstein provides a teaching of the basic concept of a main energizing coil in combination with a sensing coil. The main coil is energized with an alternating current which provides a main flux field directed through a portion of the casing or flat sheet metal. This flux field induces voltage within the metal structure which results in eddy current flow which, in turn, generates a magnetic force field proportional to the metal thickness. This field is sensed by the sensing coil to provide a relative indication of wall thickness or, in extreme circumstances, a discontinuity. Various forms of this type of sensing structure have been devised in the past. U.S. Pat. No. 3,597,678 provides another coil arrangement wherein a flux gate is used to determine variations in the magnetic properties. U.S. Pat. No. 2,315,943 discloses yet another tubular goods testing system wherein an outer main coil works in conjunction with a cylindrical slug within the tubular member. Still other coils and configurations have been used variously in prior art attempts at wall thickness measurement and these findings are listed in the Information Disclosure Statement filed concurrently herewith.